BOD REMOVALS VIA BIOLOGICAL CONTACT AND BALLASTED CLARIFICATION FOR WET WEATHER M. COTTON; D. HOLLIMAN; B. FINCHER, R. DIMASSIMO (KRUGER, INC.) Bench-scale testing was conducted to quantify the effectiveness of BOD removal utilizing an aerobic biological contact reactor and ballasted flocculation/settling. Tests were conducted at activated sludge plants. Samples were collected and mixed with Return Activated Sludge RAS targeting an MLSS concentration of 1 g/l. This mixture was aerated and true soluble and total soluble CBOD concentrations were measured over minutes. Results demonstrated that a majority of the true soluble CBOD is removed immediately upon contact with RAS. This removal was assumed to be sorption by biomass. True soluble CBOD continued to decline over the remaining test period. This was attributed to utilization by biomass for growth under firstorder kinetics. Following aeration, samples were jar tested to simulate coagulation/ballasted flocculation/settling. This removed 9 % of suspended solids resulting in removal of the majority of the particulate CBOD, sorbed soluble CBOD, and a large fraction of colloidal CBOD. Effluent Total CBOD concentrations were between 6-29 mg/l, demonstrating this treatment configuration can meet CBOD limits during wet weather events. Pilots in Knoxville, TN and Akron, OH confirm that contact tank aeration with ballasted high rate clarification will achieve 8% removal of CBOD. Effluent values of 2 mg/l Total CBOD are typical. Results have demonstrated higher CBOD removals with a higher contact tank MLSS concentration. A full scale facility in Wilson Creek, TX has produced results that validate the process' ability to achieve Total BOD removals equivalent to secondary treatment Ballasted clarification has long been accepted as a viable treatment method for the removal of solids (TSS) from wet weather wastewater flows. However, as there is no biological mechanism in a typical system, removal of Soluble BOD is minimal and total BOD removal is therefore a function of the Total BOD present as particulate. The addition of an aerated contact tank upstream of the ballasted clarification unit, where wet weather wastewater and Return Activated Sludge (RAS) are combined, has been proposed as a means to accomplish Soluble BOD (SBOD) uptake and meet the EPA s requirement of 8% Total BOD removal for secondary treatment. Materials and Methods - Bench Scale Testing The initial step in investigating this method of treatment was to conduct bench scale testing. Trials were conducted at two wastewater plants in North Carolina with different sludge ages to determine the impact of sludge age on SBOD uptake. The test procedure was as follows:
1. Sample ~3 L of Raw Wastewater. 2. Sample ~ L (vol. varied with plant) of RAS and allow to settle/thicken. 3. Add Raw Water to Bio-contact tank. 4. Start aeration and timer.. Add RAS to contact tank to achieve set MLSS. 6. Immediately sample, filter and floc filter for SCBOD, TSCBOD and TSS analyses (t=1 minute). 7. At time t =, 1, minutes, etc sample and filter (SCBOD) or floc filter (TSCBOD). 8. Conduct Ballasted jar testing on aerated samples (e.g. 1 and samples) and analyze for Total CBOD, SCBOD and TSCBOD. 9. Ballasted Floc Jar Test Procedure: Add metal salt coagulant & ballast (sand) to raw sample Mix at 3 rpm, 2 min Add polymer Mix at 2 rpm, 4 sec Settle for 2 min 1. Filter a portion of the jar test effluent for SCBOD analyses. 11. Flocculate and filter (.4 um) a portion of the jar test effluent for TSCBOD analysis. 12. Floc/Filtering for TSCBOD: ZnSO4 addition Caustic addition (to 1. ph) Settle/Filter = Colloid-free Results and Discussion Initial test results demonstrated dramatic reduction in soluble and true soluble BOD within the first minutes of aeration, indicating that the majority of SBOD removal is due to sorption alone. The more gradual decrease over the remaining time can be seen as due to respiration. SCBOD Sorption Rate per Gram Biomass per Minute Soluble CBOD True Soluble CBOD 3. Soluble CBOD Sorption (mg/min/g) 3.. 2.. 1.. Day 1. 1 1 2 Time (minutes)
When Ballasted Clarification jar tests were conducted following aeration for 1 and minutes the resulting Total CBOD and Soluble CBOD removals were > 9%. WWTP B - CBOD Removals Across BCT-HRC Bench-Scale Process Total Particulate Soluble True soluble 12 12 1 1 CBOD (mg/l) 8 6 4 68 2 2 7.7 3.7 4. 4..4 1.9 3.7 3. Raw Water Effluent @ 1' with Jar Testing Effluent @ ' with Jar Testing SCBOD removals were compared between two plants with different sludge ages (3 days vs. 12 days). The plant with the shorter sludge age (i.e. more active sludge) showed better SCBOD removals over the same time period when compared to the longer sludge age. SCBOD Removals for 2 WWTPs with Different SRTs WWTP A, 12 Day SRT, Raw TSCBOD = 44 mg/l / 18 mg/l (Day 2) WWTP B, 3 Day SRT, Raw TSCBOD = 2 mg/l 3 Soluble CBOD (mg/l) 3 2 1 CT MLSS =.88 CT MLSS = 1.1 1 1 2 Time (minutes)
Based on the bench scale testing we can conclude that the aerated contact tank in combination with ballasted flocculation will accomplish 8% removal of Total BOD. The initial rapid reduction in Soluble BOD during the aeration step can be attributed to sorption while the subsequent more gradual reduction is mainly due to respiration. The ballasted flocculation step accomplishes the removal of Particulate BOD resulting in Total BOD removals of > 9%. Knoxville, TN - Pilot Testing Pilot testing was conducted in Knoxville, TN at both the Kuwahee and 4 th Creek WWTPs in early 21. The Kuwahee plant is an activated sludge plant with a rated capacity of 44 MGD located near the University of Tennessee. The Fourth Creek plant is an activated sludge plant with a rated capacity of 1.8 MGD and is located in the suburbs of Knoxville. Pilot test goals were to remove BOD, CBOD, and TSS. Wet weather flows were simulated using a blend of raw wastewater and secondary effluent, with RAS introduced into the blended feed ahead of the contact tank. Dissolved Oxygen (DO) was monitored in the contact tank, with a value of 2. mg/l targeted. TSS was also monitored in the contact tank. As flow exited the contact tank Ferric Chloride was fed at a dose of 8-13 mg/l. An anionic dry polymer was fed in the ballasted flocculation pilot at a dose of 2.-4. mg/l. Settled water turbidity was maintained at < 2 NTU throughout the testing while operating at overflow rates of 3-4 gpm/ft2. Contact tank Mixed Liquor Suspended Solids (MLSS) levels from 4 mg/l were tested to determine the impact of MLSS concentration on BOD removals. The influent wastewater at the Kuwahee plant contained a higher industrial component and therefore a higher portion of the Total BOD was present as Soluble BOD. Initial testing at Kuwahee showed that a higher MLSS concentration in the contact tank was required to meet the 8% removal for Total BOD. Total BOD (mg/l) 3 27 2 2 17 1 1 7 Avg. Inf. TBOD Avg. Eff. TBOD % Rem. 4 8 1 12 MLSS (mg/l) 1 9 8 7 6 4 3 2 1 % Removal
During the Kuwahee study it was demonstrated that higher MLSS concentrations resulted in improved SBOD removals. Soluble BOD (mg/l) 6 4 4 3 3 2 1 Avg. Inf. SBOD Avg. Eff. SBOD % Rem. 4 8 1 12 MLSS (mg/l) 1 9 8 7 6 4 3 2 1 % Removal The Kuwahee portion of the study showed that MLSS values of greater than 1 mg/l were required to consistently meet the required 8% removal of Total BOD. SBOD removals improved as MLSS levels were increased. An average effluent Total BOD of 2 mg/l was achieved throughout the pilot. The second portion of the Knoxville study was conducted at the 4 th Creek WWTP, located in a more residential area. The soluble portion of the Total BOD was much lower at this plant which resulted in a higher RAS flow requirement to meet the selected MLSS levels. MLSS values from 4 to mg/l were again targeted during the study. Ferric Chloride was fed at 6-8 mg/l, with a cationic dry polymer dosed at 2.-4. mg/l. 4 th Creek results showed excellent Total BOD removals over all MLSS levels tested. This can be seen as mainly due to the lower SBOD levels present. Since most of the Total BOD was present as particulate BOD, this allowed the system to achieve > 9% Total BOD removals.
TotalBOD (mg/l) 3 27 2 2 17 1 1 7 Avg. Inf. TBOD Avg. Eff. TBOD % Rem. 4 6 8 1 12 MLSS (mg/l) 1 9 8 7 6 4 3 2 1 % Removal Soluble BOD removals showed improvement as MLSS levels were increased to 1 mg/l, but no additional benefit was observed at higher MLSS levels. Soluble BOD (mg/l) 6 4 4 3 3 2 1 Avg. Inf. SBOD Avg. Eff. SBOD % Rem. 4 6 8 1 12 MLSS (mg/l) 1 9 8 7 6 4 3 2 1 % Removal
Excellent Total BOD removals were observed during the 4 th Creek study primarily due to the low Soluble BOD levels present in the wet weather blend. Removals of > 9% were observed over all MLSS ranges tested, and TSS removals of > 9% were also experienced. The 4 th Creek and Kuwahee studies further validated the concept of biological contact in combination with ballasted flocculation for wet weather wastewater treatment. Akron, OH - Pilot Testing The enhanced ballasted clarification unit was piloted in Akron, OK from March December 212. The City is under a consent decree with the US EPA and State of Ohio regarding their combined sewer system and the EPA approved of the pilot study plan. The system was operated over a predetermined number of actual wet weather events with the following two objectives: 1. Meet the plant 3-day average effluent limitations which were listed as 3 mg/l Total Suspended Solids (TSS) and mg/l Carbonaceous Biochemical Oxygen Demand (cbod). 2. Demonstrate that the process could achieve > 8% Total cbod removal. During the wet weather events the pilot unit was operated with a 21 minute retention time in the contact tank. An MLSS concentration between 9 12 mg/l was targeted. The settling tank overflow rate was 4 gpm/ft2, which is considerably greater than the conventional plant. A dose of mg/l Aluminum Sulfate and 2.8 mg/l anionic polymer was fed to the system during each event.
The TSS results confirm that the pilot system achieved lower final TSS concentrations than the conventional plant secondary or final effluent. Influent Secondary Effluent Main Plant Final Eff BIOACT Eff 3 14 TSS (mg/l) 2 1 8 7 4 The pilot effluent Total CBOD concentrations were almost identical to the plant secondary effluent. Influent Main Plant Final Eff BIOACT Eff Secondary Effluent Pilot Eff 3 41 cbod (mg/l) 2 1 11 4 2
Conclusion The combination of an aerated biological contact tank and ballasted clarification has been approved for full scale implementation by several EPA regions (3, 4, and 6) as equivalent to secondary treatment for TSS and BOD removal. Bench scale testing demonstrated the viability of the process, and numerous pilot studies have confirmed its effectiveness in achieving excellent TSS and BOD removals. Full scale plants in Wilson Creek, TX and Cox Creek, MD have recently been commissioned which will provide further validation of the process as a solution to wet weather wastewater treatment issues.